The present invention relates to a method for producing orientation layers for liquid crystal displays wherein a solution of an organic prepolymer is applied to a transparent substrate and subsequently is annealed and subjected to an orientation treatment. The invention also relates to liquid crystal displays having an orientation layer produced according to this method.
In liquid crystal displays (LCDs), uniform orientation of the liquid crystals is necessary. For this reason, so-called "orientation layers" are applied to the electrodes. The orientation layers can be produced, for example, by oblique vapor deposition with SiO.sub.x. This is, however, an elaborate and relatively expensive method, and for this reason the use of orientation layers of organic materials also has been attempted.
In addition, a good seal is required in liquid crystal displays in order to ensure operational reliability of the display. In high-quality displays, sealing with glass solder, a low-melting glass, is used instead of cementing. In such glass solder-liquid crystal displays, therefore, the orientation layers must be able to withstand temperatures up to 400.degree. C. and higher without change of their morphological structure.
Organic orientation layers for liquid crystal indicators or displays consist, for example, of polyvinyl alcohol. Polyvinyl alcohol layers, however, have only very limited thermal dimensional stability and chemical resistance. For higher thermal requirements, therefore, orientation layers of polyimide are used. The orientation layers of polyimide are produced from a polyimide prepolymer. Thus, a polyamidocarboxylic acid is dissolved in a solvent and applied to the electrode layer or to the electrode base plate. After the application, the film coating is heated to higher temperatures, i.e., annealed, whereby the polyimide is formed from the polyamidocarboxylic acid. This is then followed by the orientation treatment of the polyimide resin film (see in this connection U.S. Pat. No. 4,068,923).
Because of its intrinsic coloring, however, polyimide can only be used as an orientation layer in a very small layer thicknesses. In addition, polyimide layers do not provide quality improvement functions for the glass substrate, because the polyimide itself lacks alkali resistance. Relatively expensive glass substrates with an additional SiO.sub.2 layer are, therefore, necessary. Furthermore, polyimide orientation layers are not very well suited for high-quality liquid crystal displays sealed or fused with glass solder because their thermal dimensional stability is too low.